遺伝性肥満ラットZucker fatty ratにおける脂肪分解特性:Intact fat cellにおける (-) - [<SUP>3</SUP>H] dihydroalprenololbindingの基礎的検討
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In this paper, (-) - [<SUP>3</SUP>H] dihydroalprenolol ([<SUP>3</SUP>H] DHA) binding, cyclic AMP (cAMP) and free fatty acid (FFA) accumulations of fat cells from both the genetically obese Zucker fatty rats and Wistar rats were studied for the purpose of recognizing the characteristics of lipolytic activities in the obese animals.<BR>In the experiments of [<SUP>3</SUP>H] DHA binding on intact fat cells, it was revealed that some of [<SUP>3</SUP>H] DHA bindings had properties of a β-adrenergic receptor, while the others had those of a non β-receptor, the latter showing the lack of stereospecificity and the failure to compete in a potent manner against β-adrenergic agonist. This non-β-receptor binding was effectively eliminated by addition of 10<SUP>-4</SUP>M phentolamine. This concentration of phentolamine displayed no apparent effect on the ability of isoproterenol to stimulate cAMP accumulation in the fat cells. However, even in the presence of 10<SUP>-4</SUP>M phentolamine, non-β-receptor binding was observed by increasing the concentration of [<SUP>3</SUP>H] DHA to higher than 10<SUP>-9</SUP>M. Accordingly, the experimental conditions of [<SUP>3</SUP>H] DHA binding were as follows : 3-15 × 10<SUP>4</SUP> fat cells preincubated with 10<SUP>-4</SUP>M phentolamine for 5 min were incubated for 12 min in modified Krebs-Ringer phosphate buffer (KRP, pH 7.4, 37°C), at a final volume of 1 ml containing 0.3% bovine serum albumin, 3 × 10<SUP>10</SUP>M [<SUP>3</SUP>H] DHA and competitors. The content of the tube were then rapidly filtered under vacuum through a single Whatman GF/C filter (pore size 1.2 μm). Computer-analysis of the resulting inhibition curve provided total binding, slope factor, half-maximally inhibitory concentration and non-specific binding. In this condition, [<SUP>3</SUP>H] DHA binding on intact fat cells showed the stereospecificity and the equi librium dissociation constant of (-) - and (+) -propranolol, (-) -alprenolol and (-) -isoproterenol approximate to those expected from the biological activities, displaying properties consistent with those of a β-adrenergic receptor. cAMP and FFA were determined in cell plus medium after incubation for 5 and 30 min, respectively.<BR>The data obtained from the [<SUP>3</SUP>H] DHA binding studies and cAMP measurements indicate that the equilibrium dissociation constant, maximal binding and isoproterenol-stimulated cAMP accumulations per 10<SUP>6</SUP> cells did not differ significantly between the Zucker fatty rats and Wistar rats. On the other hand, the ability of adrenalin to stimulate cAMP accumulations in fat cells showed a lower sensitivity in the Zucker fatty rats than in Wistar rats, suggesting that in the Zucker fatty rats, adrenalin; an α- and β-adrenergic agonist, and isoproterenol; a pure β-adrenergic agonist, activate adenylate cyclase through distinct mechanisms. The catecholamine-stimulated lipolysis in relation to cAMP levels was more extensive in the Zucker fatty rats than in Wistar rats, indicating the elevation of intracellular lipolytic activities in obese animals. However, basal lipolysis in 10-mo Zucker fatty rats possessing larger fat cells was equal to or slightly lower than that in Wistar rats. These findings suggest that the basal lipolysis may increase in parallel with other factors, for example, lipoprotein lipase activities, irrespective of the size of fat cell.
- 日本内分泌学会の論文
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